But you must admit, this is not the common case. If a developer regularly takes 3 months to fix every bug, then those all better be nasty heisenbeasts, because it's more likely that the developer is just slow.
It's strange that we havent figured out how to trust technical leadership to assess these things for management.
In many ways, the answer is obvious: give technical leaders economic incentives for team productivity. They will then use their expertise to actually assess relevant teams.